The prior art is replete with various solid supports or catalysts which are present in chemical reactions. However, all of these prior art references have in common the fact that the catalysts thereof are employed in a solid or slurry form, the solid material which is used being as a catalyst support or as a means for slurrying the catalyst. One particular prior art reference describes a solid support which is used specifically to adsorb and bind the reduced metallic non-oxidized insoluble solid catalysts, examples of these catalysts being solid palladium metal in a zero valence state composited on charcoal, solid nickel metal in a zero valence state composited on kieselguhr, etc., the efficacy of the process depending upon the maintenance of the metal-containing catalyst in certain reduced, insoluble, non-dissolved, solid, precipitated, dispersed forms. Using the reaction conditions set forth in this reference, the catalyst metal in the zero valence state will not dissolve, that is, it will not be a solute in the liquid reaction mixture. Furthermore any soluble metal which may be introduced into the system would be reduced to the metallic state. The solubilization of the metal from the catalysts disclosed in this reference by using an oxidizing atmosphere, as taught in the present application, would be detrimental to the methods set forth in this reference in view of the catalyst activity and economic losses which would occur due to metal solubilization.
In contradistinction to the prior art, it will be hereinafter shown in greater detail that it is possible to obtain improved yields of the desired products by effecting a condensation reaction in the presence of an oxygen-containing gas and a catalyst comprising a metal-containing compound which is soluble on the reaction medium in the presence of an inert solid material, the efficiency of the process of this invention depending upon the maintenance of the metal-containing compound in an oxidized, soluble, dissolved and non-solid form.
This invention relates to a method for the maintenance of catalytic compositions of matter in a desirable form. More specifically, the invention is concerned with a method for maintaining catalysts in a soluble form while effecting a chemical reaction in the presence of a catalyst comprising a metal-containing compound which is soluble in the reaction medium and an oxygen-containing gas, the maintenance of the metal-containing compound in soluble form being enhanced by the presence of an inert solid material.
A wide variety of chemical reactions, a particular example being a condensation reaction involving the use of an oxygen-containing gas, are catalyzed by compositions of matter comprising metal-containing compounds such as metal cations or metal complexes which are homogeneous in the reaction medium. Therefore, during the course of many of these reactions, the metal-containing catalytic compositions of matter are alternately reduced and oxidized. During this alternate reduction and oxidation, the metal-containing catalyst which is in a soluble, homogeneous state is sometimes reduced to a point where the metal is in a zero valence state and therefore may deposit out on the reactor walls or become powdery in nature. In each case, oxidation of the metal in a zero valence as a solid may not be efficient under the conditions of the reaction, and therefore the loss of the soluble metal catalyst reduces the rate of the reaction to a point which is not economically feasible to operate, or in extreme instances, will stop the reaction completely. This then necessitates replacement of the catalyst with additional amounts thereby reducing the efficiency of the reaction and increasing the cost of operation to a point, as hereinbefore set forth, may render the process economically unfeasible to operate.
It is therefore an object of this invention to provide a method for maintaining the catalyst system in a homogeneous state.
A further object of this invention is to provide a method whereby soluble metal-containing catalytic compositions of matter are maintained in said soluble form, thus permitting the reaction which employs the catalyst to proceed at an efficient rate of operation.
In one aspect an embodiment of this invention is found in a chemical condensation reaction in the presence of an oxygen-containing gas and a catalyst comprising a metal-containing compound which is soluble in the reaction medium, said chemical reaction being operated under conditions such that the metal component of said metal-containing compound is alternately reduced to a valence state of zero and oxidized back to the higher valence state of its soluble form, the improvement in the process comprising effecting said reaction in the presence of an inert solid material.
A specific embodiment of this invention is found in a palladium-containing catalyst in soluble form during a chemical condensation reaction which is effected in the presence of an oxygen-containing gas and said palladium-containing catalyst, said chemical reaction being operated under conditions such that the palladium component of said palladium-containing catalyst is alternately reduced to a valence state of zero and oxidized back to the higher valence state of its soluble form, the reaction being effected in the presence of .gamma.-alumina.
Other objects and embodiments will be found in the following further detailed description of the present invention.
As hereinbefore set forth the present invention is concerned with a process or method for maintaining metal-containing catalysts in a predetermined physical stae and preferably in a soluble state. This is advantageous when utilizing a catalyst in the state in a variety of chemical reactions. As an illustration of the difficulty which may be encountered when utilizing a metal-containing catalyst, the following equations are set forth in which R is the reacting system, R.sub.(ox) is the product or products desired and n is 1, 2 or 3 depending upon the valence of the metal portion of the catalyst. EQU R + M.sup.n.sup.+ .fwdarw. R.sub.(ox) + M.sup.o EQU 4M.sup.o + 4nH.sup.+ + nO.sub.2 .fwdarw. 4M.sup.n.sup.+ + 2nH.sub.2 O
As shown in the above reactions, it is possible that the metal-containing catalyst which is soluble in the reaction system is sometimes reduced to a metal layer which deposits on the reactor walls or to a metal powder. When this occurs, oxidation of these types of metals in a valence state of zero may not be efficient under the reaction conditions which are suitable for promoting the reaction set forth in the first equation. Thus, as hereinbefore set forth, there is a loss of the soluble metal catalyst which will retard the formation of the desired product, or in some instances, stop the reaction completely.
It has now been discovered that by effecting the reaction in the presence of an inert solid support, it will be possible to maintain the catalyst in the desired physical state, which, in many instatnces, is in solution, the presence of the support assisting in the oxidation of the metal in a valence state of zero back to the desired valence state and thus preventing the formation of metallic layer or powder, each of which is difficult to oxidize. By permitting the metal in a valence state of zero to form on the surface of the solid support, it will permit an efficient and rapid re-oxidation of the metal in a valence state of zero back to the desired higher valence state, thus allowing the metal to remain in a soluble state. In other words, the inert solid material assists the oxidation of the zero valence metal by providing a large surface area onto which the zero valence metal will adsorb, and, being so adsorbed, will itself have a larger surface area than it would have had if the inert solid material were not present. The adsorbed metal thereby achieves a higher surface-to-mass ratio than it would achieve by adsorption onto the reactor walls in the absence of the inert solid material. Therefore, the adsorbed zero valence high surface area metal is oxidized by the chemical oxidants present in the reaction system at a rate much faster than the rate of oxidation of an equivalent mass of non-adsorbed zero valence low surface area metal, precisely because of the high surface area of the former metal.
Examples of some of the catalysts which may be maintained in the soluble form comprise those transition metals which would ordinarily be reduced to a valence state of zero during reactions which they are used to catalyze. Particularly speaking, these metal-containing catalysts would include those metals of Groups VIII and IB of the Periodic Table, and specifically the salts of platinum, palladium, nickel, cobalt and silver. Illustrative examples of these salts would be platinum acetate, platinum acetylacetonate, platinum chloride, platinum bromide, palladium acetate, palladium acetylacetonate, palladium chloride, palladium bromide, nickel acetate, nickel acetylacetonate, nickel chloride, nickel bromide, cobalt acetate, cobalt acetylacetonante, cobalt chloride, cobalt bromide, silver acetate, silver acetylacetonate, silver chloride, silver bromide, etc. It is to be understood that the present invention is not necessarily limited to the maintenance of the above-mentioned catalysts in a soluble form but will extend to other catalyst salts which are soluble form and which may have the metal salts thereof reduced to zero during the specific reaction which they are employed.
The solid supports which may be utilized in the present invention comprise those which are inert or substantially inert and are not effected by the particular reaction in which they are employed. These solid supports may include both low surface area and high surface area compounds, the high surface area supports being preferred inasmuch as the metal which is deposited thereon may be more readily oxidized back to a higher valence state. Some specific examples of these solid supports will include carbon, activated carbon, diatomaceous earths such as kieselguhr, clays such as montmorillonite, kaolin, metal oxides and mixtures thereof such as alumina, either the alpha, gamma, eta, or theta variety, silica, silica-alumina, silica-magnesia-alumina, silica-alumina-magnesia, silica-alumina-zirconia, etc.
The solid support may be utilized to maintain the catalysts in a soluble state when effecting the particular reactions in either a batch or continuous type of operation. For example, when a batch type operation is used, the solid support is placed in an appropriate reaction apparatus which may comprise a flask or a pressure vessel such as an autoclave of the rotating or mixing type, the choice of said apparatus being dependent upon the particular reaction conditions which are to be employed during the reaction. Following this the catalyst is added to the reaction vessel, the feed stock is charged thereto and the reactor is sealed. In the preferred embodiment of the invention an oxygen-containing gas such as air or oxygen is also charged thereto and the reaction is allowed to proceed while employing a predetermined set of reaction conditions. These reaction conditions include elevated pressures as well as elevated temperatures, the reactor being heated to the desired operating temperature and the desired operating pressure is reached by utilizing the oxygen-containing gas such as air or oxygen. In the event that the oxygen-containinng gas is programmed to supply only a partial amount of the desired operating pressure, the remainder may be obtained by also charging thereto an inert gas such as nitrogen. The oxygen-containing gas is to be employed for the purpose of oxidizing the catalyst back to a higher valence state after said catalyst has had the metal portion thereof reduced to a valence state of zero. Upon completion of the desired residence time, the reactor is allowed to return to ambient temperature and any excess pressure is discharged to return the vessel to atmospheric pressure. The reaction mixture is recovered and the desired product is separated from any unreacted starting material and recovered by conventional means such as washing, drying, extraction, filtration, fractional distillation under reduced pressure, fractional crystallization, etc.
When employing a continuous manner of operation, the solid support of the type hereinbefore set forth in greater detail is placed in an appropriate apparatus which is maintained at the proper operating conditions of temperature and pressure. Following this the catalyst, oxygen-containing gas, and the feed stock are charged thereto through separate lines or, if so desired, the catalyst may be admixed with the feed stock and the resulting mixture charged thereto in a single stream. Upon completion of the desired residence time, the reactor effluent is continuously withdrawn and subjected to conventional means of separation whereby the desired product will be recovered and removed to storage, while any unreacted starting materials may be recycled to form a portion of the feed stock.